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Scheduled shifts in the earth's orbit should plunge the planet into a deep freeze thousands of years from now, but current changes to our atmosphere may stop it from occurring, say scientists.

Professor Thomas Crowley of the University of Edinburgh, and Dr William Hyde of the University of Toronto report in the journal Nature that the current level of carbon dioxide (CO2) in our atmosphere could negate the onset of the next Ice Age, which could occur 10,000 years from now.

But they caution that their finding is not an argument in favour of global warming, which is driving imminent and potentially far-reaching damage to the climate system.

Earth has experienced long periods of extreme cold over the billions of years of its history.

The big freezes are interspersed with "interglacial" periods of relative warmth, of the kind we have experienced since the end of the last Ice Age, around 11,000 years ago.

These climate swings have natural causes, believed to be due to changes in the earth's orbit and axis that, while minute, have a powerful effect on how much solar heat falls on the planet.

Abrupt changes

The researchers built a computer model to take a closer look at these phases of cooling and warmth.

In addition to the planetary shifts, they also factored in levels of CO2, found in tiny bubbles in ice cores, which provide an indicator of temperature spanning hundreds of thousands of years.

They found dramatic swings in climate, including changes when the earth flipped from one state to the other, which occur in a relatively short time, says Crowley.

These shifts, called "bifurcations," appear to happen in abrupt series, which is counter-intuitive to the idea that the planet cools or warms gradually.

"You had a big change about a million years ago, then a second change around 650,000 years ago, when you had bigger glaciations, then 450,000 years ago, when you started to get more repeated glaciations," says Thomas.

"What's also interesting is that the inter-glaciations also became warmer."

According to the model the next "bifurcation" would normally be due between 10,000 and 100,000 years from now.

The chill would induce a long, stable period of glaciation in the mid-latitudes, smothering Europe, Asia and parts of North America with a thick sheet of ice.

But Crowley says there is now enough CO2 in the air, as a result of fossil-fuel burning and deforestation, to offset any future cooling impacts due to orbital shift, says Crowley.

"Even the level that we have there now is more than sufficient to reach that critical state seen in the model," he said. "If we cut back [on CO2] some, that would probably still be enough."

In September, a scientific research consortium called the Global Carbon Project (GCP) said that atmospheric concentrations of CO2 reached 383 parts per million (ppm) in 2007, or 37% above pre-industrial levels.

Present concentrations are "the highest during the last 650,000 years and probably during the last 20 million years," the report says.

No green light

Crowley cautions those who would seize on the new study to say "carbon dioxide is now good, it prevents us from walking the plank into this deep glaciation."

"We don't want to give people that impression," he says. "You can't use this argument to justify [human-induced] global warming."